1. Field of the Invention
This invention relates to sinewave frequency measuring apparatus and method and more particularly to such apparatus including a frequency estimator triggered by a zero crossing of a sinewave and means for producing sample signals at a constant rate; and a method which provides and uses an approximation of the sinewave through the zero crossing, as a straight line between two sampling periods. Errors are eliminated by means of a low pass filter.
2. Related Art
The present state of the art discloses apparatus for analyzing and measuring signals. However, the state of the art is devoid of using the apparatus and method of the present invention for ascertaining the frequency of a sinewave.
For instance, U.S. Pat. No. 3,036,269, issued to Trumpy discloses a circuit for ascertaining the frequency of a supply signal. In one embodiment disclosed by Trumpy, the reference frequency is obtained from a mono-stable vibrator. The signal of unknown frequency is divided so that part of that signal is directed to a Zener diode, with the remainder of that signal being directed to continue flowing through the circuit. The Zener diode allows only the positive half of the sinewave to pass therethrough, so that it passes through to the vibrator as a square wave signal. The square wave signal unbalances the vibrator and the output of the unbalanced vibrator is then joined to the remainder signal. The combined signal then goes into a switched amplifier, whose emitted pulse is measured in width to determine the frequency of the supply signal, since a filter after the switch amplifier will sense the difference between the known frequency or reference frequency emitted by the mono-stable vibrator and the signal of unknown frequency directed to the Zener diode.
Also, U.S. Pat. No. 3,704,414 to Herbst discloses a frequency monitor for monitoring the period of a periodic waveform. The input frequency is connected to a means for detecting a zero crossing, which means includes a high gain filter whose output is differentiated by a resistor capacitor network. Accordingly, the result is a narrow pulse at the beginning of each cycle period, and at the output of the zero crossing detector. The reference frequency source is a stable frequency generator, which generates a series of pulses having a frequency very much higher than the frequency of the input source to be measured. Counters are used to count the pulses during the period between zero crossings with a first counter covering the bulk of the period and a second counter covering the remainder. The count in the second counter is periodically stored and fed to a digital-to-analog convertor which linearizes the currents to energize a meter for indicating the measurement desired. Herbst does not teach or suggest straight line estimation nor shaping of the error signal therefrom.
U.S. Pat. No. 3,743,420 to Iten discloses a method and apparatus for measuring the period of electrical signals. A time marker signal is produced by a zero crossing of an electrical signal. If the electrical signal amplitude exceeds a predetermined level during the use period prior to zero crossing, the time marker signal is produced. Therefore, the time marker signal indicates the number of cycles which have exceeded the predetermined level and the control signal indicates the total duration of those signals. The cycle duration of the time marker signal may then be determined in a counter, which is switched on and set back to zero by the control signal. Alternatively, it is possible to determine the duration of a number of input signal periods from the control signal, with the aid of the input signal periods from the control signal, and the time marker signal. The method and apparatus disclosed by Iten are especially useful for increasing the spatial resolution of flow probes.
U.S. Pat. No. 3,839,673 to Acker discloses a frequency measuring circuit, whereby an unknown frequency is determined by changing the signal to a square wave.
U.S. Pat. No. 3,984,773 to Oda discloses a pulse counting rate meter, which operates by directing a shaped pulse through a low-pass filter.
U.S. Pat. No. 4,028,985 to Merritt determines pitch or frequency of signals by use of an inhibit circuit to reject spurious peaks. Similarly, Peoples, U.S. Pat. No. 4,326,261 discloses a tone detector using a filter.
All in all, the prior art is devoid of a frequency measuring apparatus and method, whereby a period of a sinewave signal is counted at each occurrence of a change of sign from negative to positive. Errors are eliminated and noise is reduced by equally spaced output sample signals. Thus, the prior art is devoid of a zero-crossing approximation method coupled with the counting means for counting samples between zero crossings.